Examples of methods for estimating states of charge (SOC) of electric storage devices such as a secondary battery include an OCV method in which an open circuit voltage of a secondary battery is used and a current accumulation method in which charge/discharge currents of a secondary battery are accumulated (JP-A-2007-178215). If the current accumulation method is used for calculating an SOC and current accumulation is continued for considerable time, an error in estimation of SOC becomes larger because of an accumulated measurement error of a current sensor. If the current accumulation is assumed to continue for considerable time, it is preferable to use the OCV method for estimating an SOC and reset the accumulated error.
If the accumulated error is reset by estimating the SOC by the OCV method although an error in estimation of SOC by the current accumulation method is smaller than an error in estimation of SOC by the OCV method, the error in estimation of SOC may become larger.